On the Origin of Species by Means of Natural Selection (1859)
by Charles Darwin
CHAPTER I.
VARIATION UNDER DOMESTICATION.
Causes of
Variability—Effects of Habit—Correlation of
Growth—Inheritance—Character of Domestic
Varieties—Difficulty of distinguishing between Varieties and
Species—Origin of Domestic Varieties from one or more
Species—Domestic pigeons, their Differences and
Origin—Principle of Selection anciently followed, its
Effects—Methodical and Unconscious Selection—Unknown Origin
of our Domestic Productions—Circumstances favourable to Man's
power of Selection.
HEN we look to the individuals
of the same variety or sub-variety of our older cultivated plants and
animals, one of the first points which strikes us, is, that they generally
differ much more from each other, than do the individuals of any one
species or variety in a state of nature. When we reflect on the vast
diversity of the plants and animals which have been cultivated, and
which have varied during all ages under the most different climates and
treatment, I think we are driven to conclude that this greater
variability is simply due to our domestic productions having been
raised under conditions of life not so uniform as, and somewhat
different from, those to which the parent-species have been exposed
under nature. There is, also, I think, some probability in the view
propounded by Andrew Knight, that this variability may be partly
connected with excess of food. It seems pretty clear that organic
beings must be exposed during several generations to the new
conditions of life to cause any appreciable amount of variation; and
that when the organisation has once begun to vary, it generally
continues to vary for many generations. No case is on record of a
variable being ceasing to be variable under cultivation. Our oldest
cultivated plants, such as wheat, still often yield new varieties:
our oldest domesticated animals are still capable of rapid
improvement or modification.
It has been disputed at what period of time the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of
conception. Geoffroy St Hilaire's experiments show that unnatural
treatment of the embryo causes monstrosities; and monstrosities cannot
be separated by any clear line of distinction from mere
variations. But I am strongly inclined to suspect that the most
frequent cause of variability may be attributed to the male and female
reproductive elements having been affected prior to the act of
conception. Several reasons make me believe in this; but the chief one
is the remarkable effect which confinement or cultivation has on the
functions of the reproductive system; this system appearing to be far
more susceptible than any other part of the organization, to the
action of any change in the conditions of life. Nothing is more easy
than to tame an animal, and few things more difficult than to get it
to breed freely under confinement, even in the many cases when the
male and female unite. How many animals there are which will not
breed, though living long under not very close confinement in their
native country! This is generally attributed to vitiated instincts;
but how many cultivated plants display the utmost vigour, and yet
rarely or never seed! In some few such cases it has been found out
that very trifling changes, such as a little more or less water at
some particular period of growth, will determine whether or not the
plant sets a seed. I cannot here enter on the copious details which I
have collected on this curious subject; but to show how singular the
laws are which determine the reproduction of animals under
confinement, I may just mention that carnivorous animals, even from
the tropics, breed in this country pretty freely under confinement,
with the exception of the plantigrades or bear family; whereas,
carnivorous birds, with the rarest exceptions, hardly ever lay fertile
eggs. Many exotic plants have pollen utterly worthless, in the same
exact condition as in the most sterile hybrids. When, on the one hand,
we see domesticated animals and plants, though often weak and sickly,
yet breeding quite freely under confinement; and when, on the other
hand, we see individuals, though taken young from a state of nature,
perfectly tamed, long-lived, and healthy (of which I could give
numerous instances), yet having their reproductive system so seriously
affected by unperceived causes as to fail in acting, we need not be
surprised at this system, when it does act under confinement, acting
not quite regularly, and producing offspring not perfectly like their
parents or variable.
Sterility has been said to be the bane of horticulture; but on this
view we owe variability to the same cause which produces sterility;
and variability is the source of all the choicest productions of the
garden. I may add, that as some organisms will breed most freely under
the most unnatural conditions (for instance, the rabbit and ferret
kept in hutches), showing that their reproductive system has not been
thus affected; so will some animals and plants withstand domestication
or cultivation, and vary very slightly perhaps hardly more than in a
state of nature.
A long list could easily be given of "sporting plants;" by this term
gardeners mean a single bud or offset, which suddenly assumes a new
and sometimes very different character from that of the rest of the
plant. Such buds can be propagated by grafting, &c., and sometimes
by seed. These "sports" are extremely rare under nature, but far from
rare under cultivation; and in this case we see that the treatment of
the parent has affected a bud or offset, and not the ovules or
pollen. But it is the opinion of most physiologists that there is no
essential difference between a bud and an ovule in their earliest
stages of formation; so that, in fact, "sports" support my view, that
variability may be largely attributed to the ovules or pollen, or to
both, having been affected by the treatment of the parent prior to the
act of conception. These cases anyhow show that variation is not
necessarily connected, as some authors have supposed, with the act of
generation.
Seedlings from the same fruit, and the young of the same litter,
sometimes differ considerably from each other, though both the young
and the parents, as Muller has remarked, have apparently been exposed
to exactly the same conditions of life; and this shows how unimportant
the direct effects of the conditions of life are in comparison with
the laws of reproduction, and of growth, and of inheritance; for had
the action of the conditions been direct, if any of the young had
varied, all would probably have varied in the same manner. To judge
how much, in the case of any variation, we should attribute to the
direct action of heat, moisture, light, food, &c., is most
difficult: my impression is, that with animals such agencies have
produced very little direct effect, though apparently more in the case
of plants. Under this point of view, Mr. Buckman's recent experiments
on plants seem extremely valuable. When all or nearly all the
individuals exposed to certain conditions are affected in the same
way, the change at first appears to be directly due to such
conditions; but in some cases it can be shown that quite opposite
conditions produce similar changes of structure. Nevertheless some
slight amount of change may, I think, be attributed to the direct
action of the conditions of life as, in some cases, increased size
from amount of food, colour from particular kinds of food and from
light, and perhaps the thickness of fur from climate.
Habit also has a deciding influence, as in the period of flowering
with plants when transported from one climate to another. In animals
it has a more marked effect; for instance, I find in the domestic duck
that the bones of the wing weigh less and the bones of the leg more,
in proportion to the whole skeleton, than do the same bones in the
wild-duck; and I presume that this change may be safely attributed to
the domestic duck flying much less, and walking more, than its wild
parent. The great and inherited development of the udders in cows and
goats in countries where they are habitually milked, in comparison
with the state of these organs in other countries, is another instance
of the effect of use. Not a single domestic animal can be named which
has not in some country drooping ears; and the view suggested by some
authors, that the drooping is due to the disuse of the muscles of the
ear, from the animals not being much alarmed by danger, seems
probable.
There are many laws regulating variation, some few of which can be
dimly seen, and will be hereafter briefly mentioned. I will here only
allude to what may be called correlation of growth. Any change in the
embryo or larva will almost certainly entail changes in the mature
animal. In monstrosities, the correlations between quite distinct
parts are very curious; and many instances are given in Isidore
Geoffroy St Hilaire's great work on this subject. Breeders believe
that long limbs are almost always accompanied by an elongated
head. Some instances of correlation are quite whimsical; thus cats
with blue eyes are invariably deaf; colour and constitutional
peculiarities go together, of which many remarkable cases could be
given amongst animals and plants. From the facts collected by
Heusinger, it appears that white sheep and pigs are differently
affected from coloured individuals by certain vegetable
poisons. Hairless dogs have imperfect teeth; long-haired and
coarse-haired animals are apt to have, as is asserted, long or many
horns; pigeons with feathered feet have skin between their outer toes;
pigeons with short beaks have small feet, and those with long beaks
large feet. Hence, if man goes on selecting, and thus augmenting, any
peculiarity, he will almost certainly unconsciously modify other parts
of the structure, owing to the mysterious laws of the correlation of
growth.
The result of the various, quite unknown, or dimly seen laws of
variation is infinitely complex and diversified. It is well worth
while carefully to study the several treatises published on some of
our old cultivated plants, as on the hyacinth, potato, even the
dahlia, &c.; and it is really surprising to note the endless
points in structure and constitution in which the varieties and sub
varieties differ slightly from each other. The whole organization
seems to have become plastic, and tends to depart in some small degree
from that of the parental type.
Any variation which is not inherited is unimportant for us. But the
number and diversity of inheritable deviations of structure, both
those of slight and those of considerable physiological importance, is
endless. Dr. Prosper Lucas's treatise, in two large volumes, is the
fullest and the best on this subject. No breeder doubts how strong is
the tendency to inheritance: like produces like is his fundamental
belief: doubts have been thrown on this principle by theoretical
writers alone. When a deviation appears not unfrequently, and we see
it in the father and child, we cannot tell whether it may not be due
to the same original cause acting on both; but when amongst
individuals, apparently exposed to the same conditions, any very rare
deviation, due to some extraordinary combination of circumstances,
appears in the parent say, once amongst several million individuals
and it reappears in the child, the mere doctrine of chances almost
compels us to attribute its reappearance to inheritance. Every one
must have heard of cases of albinism, prickly skin, hairy bodies,
&c. appearing in several members of the same family. If strange
and rare deviations of structure are truly inherited, less strange and
commoner deviations may be freely admitted to be inheritable. Perhaps
the correct way of viewing the whole subject, would be, to look at the
inheritance of every character whatever as the rule, and
non-inheritance as the anomaly.
The laws governing inheritance are quite unknown; no one can say why
the same peculiarity in different individuals of the same species, and
in individuals of different species, is sometimes inherited and
sometimes not so; why the child often reverts in certain characters to
its grandfather or grandmother or other much more remote ancestor; why
a peculiarity is often transmitted from one sex to both sexes or to
one sex alone, more commonly but not exclusively to the like sex. It
is a fact of some little importance to us, that peculiarities
appearing in the males of our domestic breeds are often transmitted
either exclusively, or in a much greater degree, to males alone. A
much more important rule, which I think may be trusted, is that, at
whatever period of life a peculiarity first appears, it tends to
appear in the offspring at a corresponding age, though sometimes
earlier. In many cases this could not be otherwise: thus the inherited
peculiarities in the horns of cattle could appear only in the
offspring when nearly mature; peculiarities in the silkworm are known
to appear at the corresponding caterpillar or cocoon stage. But
hereditary diseases and some other facts make me believe that the rule
has a wider extension, and that when there is no apparent reason why a
peculiarity should appear at any particular age, yet that it does tend
to appear in the offspring at the same period at which it first
appeared in the parent. I believe this rule to be of the highest
importance in explaining the laws of embryology. These remarks are of
course confined to the first appearance of the peculiarity, and
not to its primary cause, which may have acted on the ovules or male
element; in nearly the same manner as in the crossed offspring from a
short-horned cow by a long-horned bull, the greater length of horn,
though appearing late in life, is clearly due to the male element.
Having alluded to the subject of reversion, I may here refer to a
statement often made by naturalists namely, that our domestic
varieties, when run wild, gradually but certainly revert in character
to their aboriginal stocks. Hence it has been argued that no
deductions can be drawn from domestic races to species in a state of
nature. I have in vain endeavoured to discover on what decisive facts
the above statement has so often and so boldly been made. There would
be great difficulty in proving its truth: we may safely conclude that
very many of the most strongly-marked domestic varieties could not
possibly live in a wild state. In many cases we do not know what the
aboriginal stock was, and so could not tell whether or not nearly
perfect reversion had ensued. It would be quite necessary, in order to
prevent the effects of intercrossing, that only a single variety
should be turned loose in its new home. Nevertheless, as our
varieties certainly do occasionally revert in some of their characters
to ancestral forms, it seems to me not improbable, that if we could
succeed in naturalising, or were to cultivate, during many
generations, the several races, for instance, of the cabbage, in very
poor soil (in which case, however, some effect would have to be
attributed to the direct action of the poor soil), that they would to
a large extent, or even wholly, revert to the wild aboriginal stock.
Whether or not the experiment would succeed, is not of great
importance for our line of argument; for by the experiment itself the
conditions of life are changed. If it could be shown that our domestic
varieties manifested a strong tendency to reversion, that is, to lose
their acquired characters, whilst kept under unchanged conditions, and
whilst kept in a considerable body, so that free intercrossing might
check, by blending together, any slight deviations of structure, in
such case, I grant that we could deduce nothing from domestic
varieties in regard to species. But there is not a shadow of evidence
in favour of this view: to assert that we could not breed our cart and
race-horses, long and short-horned cattle and poultry of various
breeds, and esculent vegetables, for an almost infinite number of
generations, would be opposed to all experience. I may add, that when
under nature the conditions of life do change, variations and
reversions of character probably do occur; but natural selection, as
will hereafter be explained, will determine how far the new characters
thus arising shall be preserved.
When we look to the hereditary varieties or races of our domestic
animals and plants, and compare them with species closely allied
together, we generally perceive in each domestic race, as already
remarked, less uniformity of character than in true species. Domestic
races of the same species, also, often have a somewhat monstrous
character; by which I mean, that, although differing from each other,
and from the other species of the same genus, in several trifling
respects, they often differ in an extreme degree in some one part,
both when compared one with another, and more especially when compared
with all the species in nature to which they are nearest allied. With
these exceptions (and with that of the perfect fertility of varieties
when crossed, a subject hereafter to be discussed), domestic races of
the same species differ from each other in the same manner as, only in
most cases in a lesser degree than, do closely-allied species of the
same genus in a state of nature. I think this must be admitted, when
we find that there are hardly any domestic races, either amongst
animals or plants, which have not been ranked by some competent judges
as mere varieties, and by other competent judges as the descendants of
aboriginally distinct species. If any marked distinction existed
between domestic races and species, this source of doubt could not so
perpetually recur. It has often been stated that domestic races do not
differ from each other in characters of generic value. I think it
could be shown that this statement is hardly correct; but naturalists
differ most widely in determining what characters are of generic
value; all such valuations being at present empirical. Moreover, on
the view of the origin of genera which I shall presently give, we have
no right to expect often to meet with generic differences in our
domesticated productions.
When we attempt to estimate the amount of structural difference
between the domestic races of the same species, we are soon involved
in doubt, from not knowing whether they have descended from one or
several parent-species. This point, if could be cleared up, would be
interesting; if, for instance, it could be shown that the greyhound,
bloodhound, terrier, spaniel, and bull-dog, which we all know
propagate their kind so truly, were the offspring of any single
species, then such facts would have great weight in making us doubt
about the immutability of the many very closely allied and natural
species for instance, of the many foxes inhabiting different quarters
of the world. I do not believe, as we shall presently see, that all
our dogs have descended from any one wild species; but, in the case of
some other domestic races, there is presumptive, or even strong,
evidence in favour of this view.
It has often been assumed that man has chosen for domestication
animals and plants having an extraordinary inherent tendency to vary,
and likewise to withstand diverse climates. I do not dispute that
these capacities have added largely to the value of most of our
domesticated productions; but how could a savage possibly know, when
he first tamed an animal, whether it would vary in succeeding
generations, and whether it would endure other climates? Has the
little variability of the ass or guinea-fowl, or the small power of
endurance of warmth by the reindeer, or of cold by the common camel,
prevented their domestication? I cannot doubt that if other animals
and plants, equal in number to our domesticated productions, and
belonging to equally diverse classes and countries, were taken from a
state of nature, and could be made to breed for an equal number of
generations under domestication, they would vary on an average as
largely as the parent species of our existing domesticated productions
have varied.
In the case of most of our anciently domesticated animals and plants,
I do not think it is possible to come to any definite conclusion,
whether they have descended from one or several species. The argument
mainly relied on by those who believe in the multiple origin of our
domestic animals is, that we find in the most ancient records, more
especially on the monuments of Egypt, much diversity in the breeds;
and that some of the breeds closely resemble, perhaps are identical
with, those still existing. Even if this latter fact were found more
strictly and generally true than seems to me to be the case, what does
it show, but that some of our breeds originated there, four or five
thousand years ago? But Mr. Horner's researches have rendered it in
some degree probable that man sufficiently civilized to have
manufactured pottery existed in the valley of the Nile thirteen or
fourteen thousand years ago; and who will pretend to say how long
before these ancient periods, savages, like those of Tierra del Fuego
or Australia, who possess a semi-domestic dog, may not have existed in
Egypt?
The whole subject must, I think, remain vague; nevertheless, I may,
without here entering on any details, state that, from geographical
and other considerations, I think it highly probable that our domestic
dogs have descended from several wild species. In regard to sheep and
goats I can form no opinion. I should think, from facts communicated
to me by Mr. Blyth, on the habits, voice, and constitution, &c., of
the humped Indian cattle, that these had descended from a different
aboriginal stock from our European cattle; and several competent
judges believe that these latter have had more than one wild parent.
With respect to horses, from reasons which I cannot give here, I am
doubtfully inclined to believe, in opposition to several authors, that
all the races have descended from one wild stock. Mr. Blyth, whose
opinion, from his large and varied stores of knowledge, I should value
more than that of almost any one, thinks that all the breeds of
poultry have proceeded from the common wild Indian fowl (Gallus
bankiva). In regard to ducks and rabbits, the breeds of which differ
considerably from each other in structure, I do not doubt that they
all have descended from the common wild duck and rabbit.
The doctrine of the origin of our several domestic races from several
aboriginal stocks, has been carried to an absurd extreme by some
authors. They believe that every race which breeds true, let the
distinctive characters be ever so slight, has had its wild prototype.
At this rate there must have existed at least a score of species of
wild cattle, as many sheep, and several goats in Europe alone, and
several even within Great Britain. One author believes that there
formerly existed in Great Britain eleven wild species of sheep
peculiar to it! When we bear in mind that Britain has now hardly one
peculiar mammal, and France but few distinct from those of Germany and
conversely, and so with Hungary, Spain, &c., but that each of
these kingdoms possesses several peculiar breeds of cattle, sheep,
&c., we must admit that many domestic breeds have originated in
Europe; for whence could they have been derived, as these several
countries do not possess a number of peculiar species as distinct
parent-stocks? So it is in India. Even in the case of the domestic
dogs of the whole world, which I fully admit have probably descended
from several wild species, I cannot doubt that there has been an
immense amount of inherited variation. Who can believe that animals
closely resembling the Italian greyhound, the bloodhound, the
bull-dog, or Blenheim spaniel, &c. so unlike all wild Canidae
ever existed freely in a state of nature? It has often been loosely
said that all our races of dogs have been produced by the crossing of
a few aboriginal species; but by crossing we can get only forms in
some degree intermediate between their parents; and if we account for
our several domestic races by this process, we must admit the former
existence of the most extreme forms, as the Italian greyhound,
bloodhound, bull-dog, &c., in the wild state. Moreover, the
possibility of making distinct races by crossing has been greatly
exaggerated. There can be no doubt that a race may be modified by
occasional crosses, if aided by the careful selection of those
individual mongrels, which present any desired character; but that a
race could be obtained nearly intermediate between two extremely
different races or species, I can hardly believe. Sir J. Sebright
expressly experimentised for this object, and failed. The offspring
from the first cross between two pure breeds is tolerably and
sometimes (as I have found with pigeons) extremely uniform, and
everything seems simple enough; but when these mongrels are crossed
one with another for several generations, hardly two of them will be
alike, and then the extreme difficulty, or rather utter hopelessness,
of the task becomes apparent. Certainly, a breed intermediate between
two very distinct breeds could not be got without extreme care
and long-continued selection; nor can I find a single case on record
of a permanent race having been thus formed.
On
the Breeds of the Domestic pigeon.—Believing
that it is always best to study some special group, I have,
after deliberation, taken up domestic pigeons. I have kept every breed
which I could purchase or obtain, and have been most kindly favoured
with skins from several quarters of the world, more especially by the
Hon. W. Elliot from India, and by the Hon. C. Murray from
Persia. Many treatises in different languages have been published on
pigeons, and some of them are very important, as being of considerably
antiquity. I have associated with several eminent fanciers, and have
been permitted to join two of the London Pigeon Clubs. The diversity
of the breeds is something astonishing. Compare the English carrier
and the short-faced tumbler, and see the wonderful difference in their
beaks, entailing corresponding differences in their skulls. The
carrier, more especially the male bird, is also remarkable from the
wonderful development of the carunculated skin about the head, and
this is accompanied by greatly elongated eyelids, very large external
orifices to the nostrils, and a wide gape of mouth. The short-faced
tumbler has a beak in outline almost like that of a finch; and the
common tumbler has the singular and strictly inherited habit of flying
at a great height in a compact flock, and tumbling in the air head
over heels. The runt is a bird of great size, with long, massive beak
and large feet; some of the sub-breeds of runts have very long necks,
others very long wings and tails, others singularly short tails. The
barb is allied to the carrier, but, instead of a very long beak, has a
very short and very broad one. The pouter has a much elongated body,
wings, and legs; and its enormously developed crop, which it glories
in inflating, may well excite astonishment and even laughter. The
turbit has a very short and conical beak, with a line of reversed
feathers down the breast; and it has the habit of continually
expanding slightly the upper part of the oesophagus. The Jacobin has
the feathers so much reversed along the back of the neck that they
form a hood, and it has, proportionally to its size, much elongated
wing and tail feathers. The trumpeter and laugher, as their names
express, utter a very different coo from the other breeds. The
fantail has thirty or even forty tail-feathers, instead of twelve or
fourteen, the normal number in all members of the great pigeon family;
and these feathers are kept expanded, and are carried so erect that in
good birds the head and tail touch; the oil-gland is quite
aborted. Several other less distinct breeds might have been specified.
In the skeletons of the several breeds, the development of the bones
of the face in length and breadth and curvature differs
enormously. The shape, as well as the breadth and length of the ramus
of the lower jaw, varies in a highly remarkable manner. The number of
the caudal and sacral vertebrae vary; as does the number of the ribs,
together with their relative breadth and the presence of processes.
The size and shape of the apertures in the sternum are highly
variable; so is the degree of divergence and relative size of the two
arms of the furcula. The proportional width of the gape of mouth, the
proportional length of the eyelids, of the orifice of the nostrils, of
the tongue (not always in strict correlation with the length of beak),
the size of the crop and of the upper part of the oesophagus; the
development and abortion of the oil-gland; the number of the primary
wing and caudal feathers; the relative length of wing and tail to each
other and to the body; the relative length of leg and of the feet; the
number of scutellae on the toes, the development of skin between the
toes, are all points of structure which are variable. The period at
which the perfect plumage is acquired varies, as does the state of the
down with which the nestling birds are clothed when hatched. The shape
and size of the eggs vary. The manner of flight differs remarkably; as
does in some breeds the voice and disposition. Lastly, in certain
breeds, the males and females have come to differ to a slight degree
from each other.
Altogether at least a score of pigeons might be chosen, which if shown
to an ornithologist, and he were told that they were wild birds, would
certainly, I think, be ranked by him as well-defined species.
Moreover, I do not believe that any ornithologist would place the
English carrier, the short-faced tumbler, the runt, the barb, pouter,
and fantail in the same genus; more especially as in each of these
breeds several truly-inherited sub-breeds, or species as he might have
called them, could be shown him.
Great as the differences are between the breeds of pigeons, I am fully
convinced that the common opinion of naturalists is correct, namely,
that all have descended from the rock-pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the
several breeds are not varieties, and have not proceeded from the
rock-pigeon, they must have descended from at least seven or eight
aboriginal stocks; for it is impossible to make the present domestic
breeds by the crossing of any lesser number: how, for instance, could
a pouter be produced by crossing two breeds unless one of the
parent-stocks possessed the characteristic enormous crop? The
supposed aboriginal stocks must all have been rock-pigeons, that is,
not breeding or willingly perching on trees. But besides C. livia,
with its geographical sub-species, only two or three other species of
rock-pigeons are known; and these have not any of the characters of
the domestic breeds. Hence the supposed aboriginal stocks must either
still exist in the countries where they were originally domesticated,
and yet be unknown to ornithologists; and this, considering their
size, habits, and remarkable characters, seems very improbable; or
they must have become extinct in the wild state. But birds breeding on
precipices, and good fliers, are unlikely to be exterminated; and the
common rock-pigeon, which has the same habits with the domestic
breeds, has not been exterminated even on several of the smaller
British islets, or on the shores of the Mediterranean. Hence the
supposed extermination of so many species having similar habits with
the rock-pigeon seems to me a very rash assumption. Moreover, the
several above-named domesticated breeds have been transported to all
parts of the world, and, therefore, some of them must have been
carried back again into their native country; but not one has ever
become wild or feral, though the dovecot-pigeon, which is the
rock-pigeon in a very slightly altered state, has become feral in
several places. Again, all recent experience shows that it is most
difficult to get any wild animal to breed freely under domestication;
yet on the hypothesis of the multiple origin of our pigeons, it must
be assumed that at least seven or eight species were so thoroughly
domesticated in ancient times by half-civilized man, as to be quite
prolific under confinement.
An argument, as it seems to me, of great weight, and applicable in
several other cases, is, that the above-specified breeds, though
agreeing generally in constitution, habits, voice, colouring, and in
most parts of their structure, with the wild rock-pigeon, yet are
certainly highly abnormal in other parts of their structure: we may
look in vain throughout the whole great family of Columbidae for a
beak like that of the English carrier, or that of the short-faced
tumbler, or barb; for reversed feathers like those of the jacobin; for
a crop like that of the pouter; for tail-feathers like those of the
fantail. Hence it must be assumed not only that half-civilized man
succeeded in thoroughly domesticating several species, but that he
intentionally or by chance picked out extraordinarily abnormal
species; and further, that these very species have since all become
extinct or unknown. So many strange contingencies seem to me
improbable in the highest degree.
Some facts in regard to the colouring of pigeons well deserve
consideration. The rock-pigeon is of a slaty-blue, and has a white
rump (the Indian sub-species, C. intermedia of Strickland, having it
bluish); the tail has a terminal dark bar, with the bases of the outer
feathers externally edged with white; the wings have two black bars:
some semi-domestic breeds and some apparently truly wild breeds have,
besides the two black bars, the wings chequered with black. These
several marks do not occur together in any other species of the whole
family. Now, in every one of the domestic breeds, taking thoroughly
well-bred birds, all the above marks, even to the white edging of the
outer tail-feathers, sometimes concur perfectly developed. Moreover,
when two birds belonging to two distinct breeds are crossed, neither
of which is blue or has any of the above-specified marks, the mongrel
offspring are very apt suddenly to acquire these characters; for
instance, I crossed some uniformly white fantails with some uniformly
black barbs, and they produced mottled brown and black birds; these I
again crossed together, and one grandchild of the pure white fantail
and pure black barb was of as beautiful a blue colour, with the white
rump, double black wing-bar, and barred and white-edged tail-feathers,
as any wild rock-pigeon! We can understand these facts, on the
well-known principle of reversion to ancestral characters, if all the
domestic breeds have descended from the rock-pigeon. But if we deny
this, we must make one of the two following highly improbable
suppositions. Either, firstly, that all the several imagined
aboriginal stocks were coloured and marked like the rock-pigeon,
although no other existing species is thus coloured and marked, so
that in each separate breed there might be a tendency to revert to the
very same colours and markings. Or, secondly, that each breed, even
the purest, has within a dozen or, at most, within a score of
generations, been crossed by the rock-pigeon: I say within a dozen or
twenty generations, for we know of no fact countenancing the belief
that the child ever reverts to some one ancestor, removed by a greater
number of generations. In a breed which has been crossed only once
with some distinct breed, the tendency to reversion to any character
derived from such cross will naturally become less and less, as in
each succeeding generation there will be less of the foreign blood;
but when there has been no cross with a distinct breed, and there is a
tendency in both parents to revert to a character, which has been lost
during some former generation, this tendency, for all that we can see
to the contrary, may be transmitted undiminished for an indefinite
number of generations. These two distinct cases are often confounded
in treatises on inheritance.
Lastly, the hybrids or mongrels from between all the domestic breeds
of pigeons are perfectly fertile. I can state this from my own
observations, purposely made on the most distinct breeds. Now, it is
difficult, perhaps impossible, to bring forward one case of the hybrid
offspring of two animals clearly distinct being themselves
perfectly fertile. Some authors believe that long-continued
domestication eliminates this strong tendency to sterility: from the
history of the dog I think there is some probability in this
hypothesis, if applied to species closely related together, though it
is unsupported by a single experiment. But to extend the hypothesis so
far as to suppose that species, aboriginally as distinct as carriers,
tumblers, pouters, and fantails now are, should yield offspring
perfectly fertile, inter se, seems to me rash in the
extreme.
From these several reasons, namely, the improbability of man having
formerly got seven or eight supposed species of pigeons to breed
freely under domestication; these supposed species being quite unknown
in a wild state, and their becoming nowhere feral; these species
having very abnormal characters in certain respects, as compared with
all other Columbidæ, though so like in most other respects to the
rock-pigeon; the blue colour and various marks occasionally appearing
in all the breeds, both when kept pure and when crossed; the mongrel
offspring being perfectly fertile;—from these several reasons,
taken together, I can feel no doubt that all our domestic breeds have
descended from the Columba livia with its geographical sub-species.
In favour of this view, I may add, firstly, that C. livia, or the
rock-pigeon, has been found capable of domestication in Europe and in
India; and that it agrees in habits and in a great number of points of
structure with all the domestic breeds. Secondly, although an English
carrier or short-faced tumbler differs immensely in certain characters
from the rock-pigeon, yet by comparing the several sub-breeds of these
breeds, more especially those brought from distant countries, we can
make an almost perfect series between the extremes of structure.
Thirdly, those characters which are mainly distinctive of each breed,
for instance the wattle and length of beak of the carrier, the
shortness of that of the tumbler, and the number of tail-feathers in
the fantail, are in each breed eminently variable; and the explanation
of this fact will be obvious when we come to treat of selection.
Fourthly, pigeons have been watched, and tended with the utmost care,
and loved by many people. They have been domesticated for thousands of
years in several quarters of the world; the earliest known record of
pigeons is in the fifth Ægyptian dynasty, about 3000 B.C., as was
pointed out to me by Professor Lepsius; but Mr. Birch informs me that
pigeons are given in a bill of fare in the previous dynasty. In the
time of the Romans, as we hear from Pliny, immense prices were given
for pigeons; "nay, they are come to this pass, that they can reckon up
their pedigree and race." Pigeons were much valued by Akber Khan in
India, about the year 1600; never less than 20,000 pigeons were taken
with the court. "The monarchs of Iran and Turan sent him some very
rare birds;" and, continues the courtly historian, "His Majesty by
crossing the breeds, which method was never practised before, has
improved them astonishingly." About this same period the Dutch were as
eager about pigeons as were the old Romans. The paramount importance
of these considerations in explaining the immense amount of variation
which pigeons have undergone, will be obvious when we treat of
Selection. We shall then, also, see how it is that the breeds so often
have a somewhat monstrous character. It is also a most favourable
circumstance for the production of distinct breeds, that male and
female pigeons can be easily mated for life; and thus different breeds
can be kept together in the same aviary.
I have discussed the probable origin of domestic pigeons at some, yet
quite insufficient, length; because when I first kept pigeons and
watched the several kinds, knowing well how true they bred, I felt
fully as much difficulty in believing that they could ever have
descended from a common parent, as any naturalist could in coming to a
similar conclusion in regard to the many species of finches, or other
large groups of birds, in nature. One circumstance has struck me much;
namely, that all the breeders of the various domestic animals and the
cultivators of plants, with whom I have ever conversed, or whose
treatises I have read, are firmly convinced that the several breeds to
which each has attended, are descended from so many aboriginally
distinct species. Ask, as I have asked, a celebrated raiser of
Hereford cattle, whether his cattle might not have descended from long
horns, and he will laugh you to scorn. I have never met a pigeon, or
poultry, or duck, or rabbit fancier, who was not fully convinced that
each main breed was descended from a distinct species. Van Mons, in
his treatise on pears and apples, shows how utterly he disbelieves
that the several sorts, for instance a Ribston-pippin or Codlin-apple,
could ever have proceeded from the seeds of the same tree. Innumerable
other examples could be given. The explanation, I think, is simple:
from long-continued study they are strongly impressed with the
differences between the several races; and though they well know that
each race varies slightly, for they win their prizes by selecting such
slight differences, yet they ignore all general arguments, and refuse
to sum up in their minds slight differences accumulated during many
successive generations. May not those naturalists who, knowing far
less of the laws of inheritance than does the breeder, and knowing no
more than he does of the intermediate links in the long lines of
descent, yet admit that many of our domestic races have descended from
the same parents—may they not learn a lesson of caution, when
they deride the idea of species in a state of nature being lineal
descendants of other species?
Selection.—Let
us now briefly consider the steps by which domestic races have
been produced, either from one or from several allied species. Some
little effect may, perhaps, be attributed to the direct action of the
external conditions of life, and some little to habit; but he would be
a bold man who would account by such agencies for the differences of a
dray and race horse, a greyhound and bloodhound, a carrier and tumbler
pigeon. One of the most remarkable features in our domesticated races
is that we see in them adaptation, not indeed to the animal's or
plant's own good, but to man's use or fancy. Some variations useful to
him have probably arisen suddenly, or by one step; many botanists, for
instance, believe that the fuller's teazle, with its hooks, which
cannot be rivalled by any mechanical contrivance, is only a variety of
the wild Dipsacus; and this amount of change may have suddenly arisen
in a seedling. So it has probably been with the turnspit dog; and this
is known to have been the case with the ancon sheep. But when we
compare the dray-horse and race-horse, the dromedary and camel, the
various breeds of sheep fitted either for cultivated land or mountain
pasture, with the wool of one breed good for one purpose, and that of
another breed for another purpose; when we compare the many breeds of
dogs, each good for man in very different ways; when we compare the
gamecock, so pertinacious in battle, with other breeds so little
quarrelsome, with "everlasting layers" which never desire to sit, and
with the bantam so small and elegant; when we compare the host of
agricultural, culinary, orchard, and flower-garden races of plants,
most useful to man at different seasons and for different purposes, or
so beautiful in his eyes, we must, I think, look further than to mere
variability. We cannot suppose that all the breeds were suddenly
produced as perfect and as useful as we now see them; indeed, in
several cases, we know that this has not been their history. The key
is man's power of accumulative selection: nature gives successive
variations; man adds them up in certain directions useful to him. In
this sense he may be said to make for himself useful breeds.
The great power of this principle of selection is not hypothetical.
It is certain that several of our eminent breeders have, even within a
single lifetime, modified to a large extent some breeds of cattle and
sheep. In order fully to realise what they have done, it is almost
necessary to read several of the many treatises devoted to this
subject, and to inspect the animals. Breeders habitually speak of an
animal's organisation as something quite plastic, which they can model
almost as they please. If I had space I could quote numerous passages
to this effect from highly competent authorities. Youatt, who was
probably better acquainted with the works of agriculturalists than
almost any other individual, and who was himself a very good judge of
an animal, speaks of the principle of selection as "that which enables
the agriculturist, not only to modify the character of his flock, but
to change it altogether. It is the magician's wand, by means of which
he may summon into life whatever form and mould he pleases." Lord
Somerville, speaking of what breeders have done for sheep,
says:—"It would seem as if they had chalked out upon a wall a form
perfect in itself, and then had given it existence." That most skilful
breeder, Sir John Sebright, used to say, with respect to pigeons, that
"he would produce any given feather in three years, but it would take
him six years to obtain head and beak." In Saxony the importance of
the principle of selection in regard to merino sheep is so fully
recognised, that men follow it as a trade: the sheep are placed on a
table and are studied, like a picture by a connoisseur; this is done
three times at intervals of months, and the sheep are each time marked
and classed, so that the very best may ultimately be selected for
breeding.
What English breeders have actually effected is proved by the enormous
prices given for animals with a good pedigree; and these have now been
exported to almost every quarter of the world. The improvement is by
no means generally due to crossing different breeds; all the best
breeders are strongly opposed to this practice, except sometimes
amongst closely allied sub-breeds. And when a cross has been made,
the closest selection is far more indispensable even than in ordinary
cases. If selection consisted merely in separating some very distinct
variety, and breeding from it, the principle would be so obvious as
hardly to be worth notice; but its importance consists in the great
effect produced by the accumulation in one direction, during
successive generations, of differences absolutely inappreciable by an
uneducated eye—differences which I for one have vainly attempted
to appreciate. Not one man in a thousand has accuracy of eye and
judgement sufficient to become an eminent breeder. If gifted with
these qualities, and he studies his subject for years, and devotes his
lifetime to it with indomitable perseverance, he will succeed, and may
make great improvements; if he wants any of these qualities, he will
assuredly fail. Few would readily believe in the natural capacity and
years of practice requisite to become even a skilful pigeon-fancier.
The same principles are followed by horticulturists; but the
variations are here often more abrupt. No one supposes that our
choicest productions have been produced by a single variation from the
aboriginal stock. We have proofs that this is not so in some cases, in
which exact records have been kept; thus, to give a very trifling
instance, the steadily-increasing size of the common gooseberry may be
quoted. We see an astonishing improvement in many florists' flowers,
when the flowers of the present day are compared with drawings made
only twenty or thirty years ago. When a race of plants is once pretty
well established, the seed-raisers do not pick out the best plants,
but merely go over their seed-beds, and pull up the "rogues," as they
call the plants that deviate from the proper standard. With animals
this kind of selection is, in fact, also followed; for hardly any one
is so careless as to allow his worst animals to breed.
In
regard to plants, there is another means of observing the accumulated
effects of selection—namely, by comparing the diversity of
flowers in the different varieties of the same species in the
flower-garden; the diversity of leaves, pods, or tubers, or whatever
part is valued, in the kitchen-garden, in comparison with the flowers
of the same varieties; and the diversity of fruit of the same species
in the orchard, in comparison with the leaves and flowers of the same
set of varieties. See how different the leaves of the cabbage are, and
how extremely alike the flowers; how unlike the flowers of the
heartsease are, and how alike the leaves; how much the fruit of the
different kinds of gooseberries differ in size, colour, shape, and
hairiness, and yet the flowers present very slight differences. It is
not that the varieties which differ largely in some one point do not
differ at all in other points; this is hardly ever, perhaps never, the
case. The laws of correlation of growth, the importance of which
should never be overlooked, will ensure some differences; but, as a
general rule, I cannot doubt that the continued selection of slight
variations, either in the leaves, the flowers, or the fruit, will
produce races differing from each other chiefly in these characters.
It may be objected that the principle of selection has been reduced to
methodical practice for scarcely more than three-quarters of a
century; it has certainly been more attended to of late years, and
many treatises have been published on the subject; and the result, I
may add, has been, in a corresponding degree, rapid and important. But
it is very far from true that the principle is a modern discovery. I
could give several references to the full acknowledgement of the
importance of the principle in works of high antiquity. In rude and
barbarous periods of English history choice animals were often
imported, and laws were passed to prevent their exportation: the
destruction of horses under a certain size was ordered, and this may
be compared to the "roguing" of plants by nurserymen. The principle
of selection I find distinctly given in an ancient Chinese
encyclopaedia. Explicit rules are laid down by some of the Roman
classical writers. From passages in Genesis, it is clear that the
colour of domestic animals was at that early period attended to.
Savages now sometimes cross their dogs with wild canine animals, to
improve the breed, and they formerly did so, as is attested by
passages in Pliny. The savages in South Africa match their draught
cattle by colour, as do some of the Esquimaux their teams of dogs.
Livingstone shows how much good domestic breeds are valued by the
negroes of the interior of Africa who have not associated with
Europeans. Some of these facts do not show actual selection, but they
show that the breeding of domestic animals was carefully attended to
in ancient times, and is now attended to by the lowest savages. It
would, indeed, have been a strange fact, had attention not been paid
to breeding, for the inheritance of good and bad qualities is so
obvious.
At the present time, eminent breeders try by methodical selection,
with a distinct object in view, to make a new strain or sub-breed,
superior to anything existing in the country. But, for our purpose, a
kind of Selection, which may be called Unconscious, and which results
from every one trying to possess and breed from the best individual
animals, is more important. Thus, a man who intends keeping pointers
naturally tries to get as good dogs as he can, and afterwards breeds
from his own best dogs, but he has no wish or expectation of
permanently altering the breed. Nevertheless I cannot doubt that this
process, continued during centuries, would improve and modify any
breed, in the same way as Bakewell, Collins, &c., by this very
same process, only carried on more methodically, did greatly modify,
even during their own lifetimes, the forms and qualities of their
cattle. Slow and insensible changes of this kind could never be
recognised unless actual measurements or careful drawings of the
breeds in question had been made long ago, which might serve for
comparison. In some cases, however, unchanged or but little changed
individuals of the same breed may be found in less civilised
districts, where the breed has been less improved. There is reason to
believe that King Charles's spaniel has been unconsciously modified to
a large extent since the time of that monarch. Some highly competent
authorities are convinced that the setter is directly derived from the
spaniel, and has probably been slowly altered from it. It is known
that the English pointer has been greatly changed within the last
century, and in this case the change has, it is believed, been chiefly
effected by crosses with the fox-hound; but what concerns us is, that
the change has been effected unconsciously and gradually, and yet so
effectually, that, though the old Spanish pointer certainly came from
Spain, Mr. Barrow has not seen, as I am informed by him, any native dog
in Spain like our pointer.
By a similar process of selection, and by careful training, the whole
body of English racehorses have come to surpass in fleetness and size
the parent Arab stock, so that the latter, by the regulations for the
Goodwood Races, are favoured in the weights they carry. Lord Spencer
and others have shown how the cattle of England have increased in
weight and in early maturity, compared with the stock formerly kept in
this country. By comparing the accounts given in old pigeon treatises
of carriers and tumblers with these breeds as now existing in Britain,
India, and Persia, we can, I think, clearly trace the stages through
which they have insensibly passed, and come to differ so greatly from
the rock-pigeon.
Youatt gives an excellent illustration of the effects of a course of
selection, which may be considered as unconsciously followed, in so
far that the breeders could never have expected or even have wished to
have produced the result which ensued—namely, the production of two
distinct strains. The two flocks of Leicester sheep kept by Mr. Buckley
and Mr. Burgess, as Mr. Youatt remarks, "have been purely bred from the
original stock of Mr. Bakewell for upwards of fifty years. There is
not a suspicion existing in the mind of any one at all acquainted with
the subject that the owner of either of them has deviated in any one
instance from the pure blood of Mr. Bakewell's flock, and yet the
difference between the sheep possessed by these two gentlemen is so
great that they have the appearance of being quite different
varieties."
If there exist savages so barbarous as never to think of the inherited
character of the offspring of their domestic animals, yet any one
animal particularly useful to them, for any special purpose, would be
carefully preserved during famines and other accidents, to which
savages are so liable, and such choice animals would thus generally
leave more offspring than the inferior ones; so that in this case
there would be a kind of unconscious selection going on. We see the
value set on animals even by the barbarians of Tierra del Fuego, by
their killing and devouring their old women, in times of dearth, as of
less value than their dogs.
In plants the same gradual process of improvement, through the
occasional preservation of the best individuals, whether or not
sufficiently distinct to be ranked at their first appearance as
distinct varieties, and whether or not two or more species or races
have become blended together by crossing, may plainly be recognised in
the increased size and beauty which we now see in the varieties of the
heartsease, rose, pelargonium, dahlia, and other plants, when compared
with the older varieties or with their parent-stocks. No one would
ever expect to get a first-rate heartsease or dahlia from the seed of
a wild plant. No one would expect to raise a first-rate melting pear
from the seed of a wild pear, though he might succeed from a poor
seedling growing wild, if it had come from a garden-stock. The pear,
though cultivated in classical times, appears, from Pliny's
description, to have been a fruit of very inferior quality. I have
seen great surprise expressed in horticultural works at the wonderful
skill of gardeners, in having produced such splendid results from such
poor materials; but the art, I cannot doubt, has been simple, and, as
far as the final result is concerned, has been followed almost
unconsciously. It has consisted in always cultivating the best known
variety, sowing its seeds, and, when a slightly better variety has
chanced to appear, selecting it, and so onwards. But the gardeners of
the classical period, who cultivated the best pear they could procure,
never thought what splendid fruit we should eat; though we owe our
excellent fruit, in some small degree, to their having naturally
chosen and preserved the best varieties they could anywhere find.
A large amount of change in our cultivated plants, thus slowly and
unconsciously accumulated, explains, as I believe, the well-known
fact, that in a vast number of cases we cannot recognise, and
therefore do not know, the wild parent-stocks of the plants which have
been longest cultivated in our flower and kitchen gardens. If it has
taken centuries or thousands of years to improve or modify most of our
plants up to their present standard of usefulness to man, we can
understand how it is that neither Australia, the Cape of Good Hope,
nor any other region inhabited by quite uncivilised man, has afforded
us a single plant worth culture. It is not that these countries, so
rich in species, do not by a strange chance possess the aboriginal
stocks of any useful plants, but that the native plants have not been
improved by continued selection up to a standard of perfection
comparable with that given to the plants in countries anciently
civilised.
In regard to the domestic animals kept by uncivilised man, it should
not be overlooked that they almost always have to struggle for their
own food, at least during certain seasons. And in two countries very
differently circumstanced, individuals of the same species, having
slightly different constitutions or structure, would often succeed
better in the one country than in the other, and thus by a process of
"natural selection," as will hereafter be more fully explained, two
sub-breeds might be formed. This, perhaps, partly explains what has
been remarked by some authors, namely, that the varieties kept by
savages have more of the character of species than the varieties kept
in civilised countries.
On the view here given of the all-important part which selection by
man has played, it becomes at once obvious, how it is that our
domestic races show adaptation in their structure or in their habits
to man's wants or fancies. We can, I think, further understand the
frequently abnormal character of our domestic races, and likewise
their differences being so great in external characters and relatively
so slight in internal parts or organs. Man can hardly select, or only
with much difficulty, any deviation of structure excepting such as is
externally visible; and indeed he rarely cares for what is internal.
He can never act by selection, excepting on variations which are first
given to him in some slight degree by nature. No man would ever try to
make a fantail, till he saw a pigeon with a tail developed in some
slight degree in an unusual manner, or a pouter till he saw a pigeon
with a crop of somewhat unusual size; and the more abnormal or unusual
any character was when it first appeared, the more likely it would be
to catch his attention. But to use such an expression as trying to
make a fantail, is, I have no doubt, in most cases, utterly incorrect.
The man who first selected a pigeon with a slightly larger tail, never
dreamed what the descendants of that pigeon would become through
long-continued, partly unconscious and partly methodical selection.
Perhaps the parent bird of all fantails had only fourteen
tail-feathers somewhat expanded, like the present Java fantail, or
like individuals of other and distinct breeds, in which as many as
seventeen tail-feathers have been counted. Perhaps the first
pouter-pigeon did not inflate its crop much more than the turbit now
does the upper part of its œsophagus,—a habit which is
disregarded by all fanciers, as it is not one of the points of the
breed.
Nor let it be thought that some great deviation of structure would be
necessary to catch the fancier's eye: he perceives extremely small
differences, and it is in human nature to value any novelty, however
slight, in one's own possession. Nor must the value which would
formerly be set on any slight differences in the individuals of the
same species, be judged of by the value which would now be set on
them, after several breeds have once fairly been established. Many
slight differences might, and indeed do now, arise amongst pigeons,
which are rejected as faults or deviations from the standard of
perfection of each breed. The common goose has not given rise to any
marked varieties; hence the Thoulouse and the common breed, which
differ only in colour, that most fleeting of characters, have lately
been exhibited as distinct at our poultry-shows.
I think these
views further explain what has sometimes been noticed—namely
that we know nothing about the origin or history of any of
our domestic breeds. But, in fact, a breed, like a dialect of a
language, can hardly be said to have had a definite origin. A man
preserves and breeds from an individual with some slight deviation of
structure, or takes more care than usual in matching his best animals
and thus improves them, and the improved individuals slowly spread in
the immediate neighbourhood. But as yet they will hardly have a
distinct name, and from being only slightly valued, their history will
be disregarded. When further improved by the same slow and gradual
process, they will spread more widely, and will get recognised as
something distinct and valuable, and will then probably first receive
a provincial name. In semi-civilised countries, with little free
communication, the spreading and knowledge of any new sub-breed will
be a slow process. As soon as the points of value of the new sub-breed
are once fully acknowledged, the principle, as I have called it, of
unconscious selection will always tend,—perhaps more at one period
than at another, as the breed rises or falls in fashion,—perhaps
more in one district than in another, according to the state of
civilisation of the inhabitants,—slowly to add to the characteristic
features of the breed, whatever they may be. But the chance will be
infinitely small of any record having been preserved of such slow,
varying, and insensible changes.
I must now say a few words on the circumstances, favourable, or the
reverse, to man's power of selection. A high degree of variability is
obviously favourable, as freely giving the materials for selection to
work on; not that mere individual differences are not amply
sufficient, with extreme care, to allow of the accumulation of a large
amount of modification in almost any desired direction. But as
variations manifestly useful or pleasing to man appear only
occasionally, the chance of their appearance will be much increased by
a large number of individuals being kept; and hence this comes to be
of the highest importance to success. On this principle Marshall has
remarked, with respect to the sheep of parts of Yorkshire, that "as
they generally belong to poor people, and are mostly in small
lots, they never can be improved." On the other hand, nurserymen,
from raising large stocks of the same plants, are generally far more
successful than amateurs in getting new and valuable varieties. The
keeping of a large number of individuals of a species in any country
requires that the species should be placed under favourable conditions
of life, so as to breed freely in that country. When the individuals
of any species are scanty, all the individuals, whatever their quality
may be, will generally be allowed to breed, and this will effectually
prevent selection. But probably the most important point of all, is,
that the animal or plant should be so highly useful to man, or so much
valued by him, that the closest attention should be paid to even the
slightest deviation in the qualities or structure of each
individual. Unless such attention be paid nothing can be effected. I
have seen it gravely remarked, that it was most fortunate that the
strawberry began to vary just when gardeners began to attend closely
to this plant. No doubt the strawberry had always varied since it was
cultivated, but the slight varieties had been neglected. As soon,
however, as gardeners picked out individual plants with slightly
larger, earlier, or better fruit, and raised seedlings from them, and
again picked out the best seedlings and bred from them, then, there
appeared (aided by some crossing with distinct species) those many
admirable varieties of the strawberry which have been raised during
the last thirty or forty years.
In the case of animals with separate sexes, facility in preventing
crosses is an important element of success in the formation of new
races,—at least, in a country which is already stocked with other
races. In this respect enclosure of the land plays a part. Wandering
savages or the inhabitants of open plains rarely possess more than one
breed of the same species. Pigeons can be mated for life, and this is
a great convenience to the fancier, for thus many races may be kept
true, though mingled in the same aviary; and this circumstance must
have largely favoured the improvement and formation of new
breeds. Pigeons, I may add, can be propagated in great numbers and at
a very quick rate, and inferior birds may be freely rejected, as when
killed they serve for food. On the other hand, cats, from their
nocturnal rambling habits, cannot be matched, and, although so much
valued by women and children, we hardly ever see a distinct breed kept
up; such breeds as we do sometimes see are almost always imported from
some other country, often from islands. Although I do not doubt that
some domestic animals vary less than others, yet the rarity or absence
of distinct breeds of the cat, the donkey, peacock, goose, &c.,
may be attributed in main part to selection not having been brought
into play: in cats, from the difficulty in pairing them; in donkeys,
from only a few being kept by poor people, and little attention paid
to their breeding; in peacocks, from not being very easily reared and
a large stock not kept; in geese, from being valuable only for two
purposes, food and feathers, and more especially from no pleasure
having been felt in the display of distinct breeds.
To sum up on the origin of our Domestic Races of animals and plants. I
believe that the conditions of life, from their action on the
reproductive system, are so far of the highest importance as causing
variability. I do not believe that variability is an inherent and
necessary contingency, under all circumstances, with all organic
beings, as some authors have thought. The effects of variability are
modified by various degrees of inheritance and of
reversion. Variability is governed by many unknown laws, more
especially by that of correlation of growth. Something may be
attributed to the direct action of the conditions of life. Something
must be attributed to use and disuse. The final result is thus
rendered infinitely complex. In some cases, I do not doubt that the
intercrossing of species, aboriginally distinct, has played an
important part in the origin of our domestic productions. When in any
country several domestic breeds have once been established, their
occasional intercrossing, with the aid of selection, has, no doubt,
largely aided in the formation of new sub-breeds; but the importance
of the crossing of varieties has, I believe, been greatly exaggerated,
both in regard to animals and to those plants which are propagated by
seed. In plants which are temporarily propagated by cuttings, buds,
&c., the importance of the crossing both of distinct species and
of varieties is immense; for the cultivator here quite disregards the
extreme variability both of hybrids and mongrels, and the frequent
sterility of hybrids; but the cases of plants not propagated by seed
are of little importance to us, for their endurance is only temporary.
Over all these causes of Change I am convinced that the accumulative
action of Selection, whether applied methodically and more quickly, or
unconsciously and more slowly, but more efficiently, is by far the
predominant Power.
[ Charles Darwin,
On
the Origin Of Species: A Facsimile of the First Edition,
Cambridge, Massachusetts: Harvard University Press, 1964, pp. 7-43. ]
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